The present invention relates to a rigid nozzle wall for expansion ramps and hot gas nozzles, comprising an outer support structure on the non-hot gas wetted side and a multilayer inner structure with spaced-apart cooling ducts on the hot gas wetted side, and a method for manufacturing such a nozzle wall so that the wall will largely remain free of distortion under operational conditions.
For reusable spaceplanes (e.g. the SANGER project), hybrid engines composed of diverse propulsion systems are envisaged. To obtain high thrust and facilitate switchover between engine types, two-dimensional (2D) nozzles of rectangular cross section have proved especially useful. The walls of such nozzles come under high compressive forces and temperatures. Such compressive forces cause high bending moments in the plane walls of 2D nozzles, unlike in the walls of circular nozzles. This may induce warping and straining in the nozzle and so jeopardize its proper function. The situation is aggravated by the bimetallic effect caused by differences in temperatures across the wall. To prevent thrust losses and leakage flows, therefore, dimensionally stable, cooled walls are needed.
DE-PS 40 15 204 discloses a multilayer nozzle wall, where an inner cooling layer is joined to an outer support structure through a cast-in intermediate layer. In this arrangement, temperature-induced elongations between the hot and cold sides cause undesirable warping of the layers.
It is a particular object of the present invention to provide an arrangement in which the inner structure consists of a hot-gas wetted thermally conductive layer and a heat-resistant sliding layer, the cooling ducts are imbedded in the thermally conductive layer and the thermally conductive layer is flexibly joined to the support structure through several fasteners extending through the sliding layer.
An advantage of the present invention is that by the flexible connection of the thermally conductive layer to the support structure, low-stress thermal expansion of the thermally conductive layer is ensured without permitting the differing layer temperatures to cause warping. Segregating the layers in this manner prevents warp-inducing shear stresses from being transferred between layers or from being generated at all. Transfer of the gas pressure forces is effected by the fasteners and by the sliding layer, which simultaneously forms a thermal barrier and protects the support structure from excessive temperatures.
The arrangement of the present invention also reduces the amount of coolant required for the cooling ducts in the thermally conductive layer. A plurality of fasteners simultaneously assures the thermally conductive layer conforms to the contour of the rigid support structure. To facilitate replacement of the thermally conductive layer when damaged, or for experimental purposes, the fasteners can be disengaged from the support structure, considering that the sliding layer is not fixedly connected to the adjacent layers.
In a preferred embodiment of the present invention the cooling ducts are formed by cooling tubes to enable the nozzle wall to be manufactured in a simple and low-cost manner.
In a further embodiment of the present invention, the fasteners are each connected in the thermally conductive layer to a cooling duct or tube so as to reliably and firmly mate the cooling layer to the support structure. To this end an especially advantageous arrangement is one in which the fasteners take the shape of hooks for improved anchorage in the thermally conductive layer. In this arrangement, the fasteners preferably envelope the cooling ducts.
In a presently preferred embodiment of the present invention the fasteners take the shape of bent ends of wire to facilitate manufacture and lower cost. In another presently preferred aspect of the present invention the fasteners are brazed to the support structure. This type of connection is easy to make and additionally facilitates replacement of the thermally conductive layer by heating the braze alloy to melting temperature and thereby permitting the fasteners to be detached from the support structure.
In yet a further embodiment of the present invention the sliding layer is made from a ceramic granular material to achieve favorable sliding properties at high-temperature resistance and concurrent thermal insulation relative to the support structure. This arrangement also facilitates manufacture in that the granular material can simply be filled into the space between the support structure and the thermally conductive layer.
In still a further preferred embodiment of the present invention the thermally conductive layer is made of oxygen-free copper to prevent reaction of the coolant, normally hydrogen, with the thermally conductive layer. Warping of the thermally conductive layer as a result of differing thicknesses of layer above and below the tubes can be prevented by dimensioning the layer, as measured radially above and below the cooling ducts, to be approximately the same.
In an alternative embodiment of the present invention the support structure is provided with webs to provide greater stiffness, especially bending stiffness of the nozzle wall.
In areas under high thermal load, as perhaps near the nozzle throat, it may also be necessary to provide added cooling capacity. In a further embodiment of the present invention, therefore, the cooling ducts branch out into several smaller ducts to reduce the heat transfer resistance, which largely determines the amount of heat being dissipated.
A novel method for manufacturing a nozzle wall having the above features and advantages is described herein and provides an advantage in that the successive process operations can be automated and allow quality inspection between operations.